1. Field of the Invention
The present invention relates to an electronic watermarking method, a digital information distribution system using such a method, and a storage medium storing a program of such a method. More particularly, the invention relates to electronic watermarking techniques for protecting a copyright of digital information such as moving image data, still image data, voice data, computer data, and computer programs, and to a digital information distribution system using such techniques suitable for use with a multimedia network.
2. Related Background Art
With recent developments of computer networks and widespread use of inexpensive and high performance computers, electronic commerce of selling and buying commodities on networks is prevalent. Commodities sold and bought in electronic commerce may be digital data including images.
However, digital data has a general property that a number of perfect copies of the digital data can be made easily. There is therefore a possibility that a user who bought a commodity of digital data illegally makes copies of the same quality as the original digital data and redistributes them to other users. In this case, a copyright holder of the commodity or an agent (sales agent) entrusted by the copyright holder cannot receive a proper countervalue to be paid for the commodity, and the copyright is infringed.
If the copyright holder or sales agent (hereinafter both are collectively called a server which can legally distribute commodities of digital data) sends a commodity to a buyer once, it is impossible to entirely prevent illegal copies of the commodity. As techniques for effectively protecting copyright instead of directly preventing illegal copies, techniques using a so-called xe2x80x9celectronic watermarkxe2x80x9d have been proposed.
Electronic watermark techniques embed copyright information of digital data and user information of a buyer in original digital data itself by processing the original digital data, to thereby identify the person who illegally copied and redistributed the digital data.
A conventional system using electronic watermark techniques relies upon an assumption that a server is a perfectly trustful facility. If the server is not trustful and performs illegal operations, even a user not making illegal copies in a conventional system may be made chargeable with illegal copies, by the server.
This claim by the server of a conventional electronic watermark system cannot be refuted by users because of the following reason. The server embeds user information in original digital data (in the following, image data is used by way of example) in order to identify the user. Therefore, if the server illegally distributes copies by intentionally embedding information of a particular user, this particular user is judged as illegal.
It is an object of the present invention to solve the above-described problem and provide reliable prevention of illegal use of digital data as well as reduction of an amount of communication and calculation necessary for such reliable prevention.
According to one aspect of the invention, an electronic watermarking method comprises: a first step of generating a plurality of pieces of information having different electronic watermark information; a second step of generating a plurality of pieces of encrypted information through encryption of each of the plurality of pieces of information with each of a plurality of independent encryption keys; a third step of generating a plurality of encrypted encryption keys through encryption of each independent encryption key with each of the other independent encryption keys; a fourth step of transmitting the plurality of pieces of encrypted information and the plurality of encrypted encryption keys to a user, the first to fourth steps being executed by a server; and a fifth step of selecting, for each piece of information, a pair comprising a piece of the plurality of pieces of encrypted information and a corresponding encrypted encryption key, the fifth step being executed by the user, wherein only some of the encryption keys are multiplied by random numbers.
Another feature of the electronic watermarking method of the invention, is that the server generates an encrypted random number through encryption of a random number, reserves the encrypted random number, and transmits the encrypted random number to the user.
Another feature of the electronic watermarking method of the invention, is that a validity of an electronic watermark is verified.
Another feature of the electronic watermarking method of the invention, is that a validity of an electronic watermark is verified repetitively.
According to another aspect of the invention, an electronic watermarking method is provided in which a validity of electronic watermark information of a user embedded through an electronic watermarking process is guaranteed through an encryption process, wherein the electronic watermark information of the user is inspected.
Another feature of the electronic watermarking method of the invention, is that a signature of the user is verified by using an anonymous public key with a certificate issued by a certification facility.
Another feature of the electronic watermarking method of the invention, is that an image format is used which has a header field storing information to be used for inspecting the encryption process and/or electronic watermark embedding process.
According to another aspect of the invention, a digital information distribution system comprises: first means for generating a plurality of pieces of information having different electronic watermark information; second means for generating a plurality of pieces of encrypted information through encryption of each of the plurality of pieces of information with each of a plurality of independent encryption keys; third means for generating a plurality of encrypted encryption keys through encryption of each independent encryption key with each of the other independent encryption keys; fourth means for transmitting the plurality of pieces of encrypted information and the plurality of encrypted encryption keys to a user, a server being provided with the first to fourth means; fifth means for selecting, for each piece of information, a pair comprising a piece of the plurality of pieces of encrypted information and a corresponding encrypted encryption key, the user being provided with the fifth means; and sixth means for multiplying only some of the encryption keys; by random numbers.
Another feature of the digital information distribution system is that the system further comprises seventh means for generating an encrypted random number through encryption of a random number, reserving the encrypted random number, and transmitting the encrypted random number to the user, the server being provided with the seventh means.
Another feature of the digital information distribution system is that the system further comprises eighth means for verifying a validity of an electronic watermark.
Another feature of the digital information distribution system is that a validity of an electronic watermark is verified repetitively.
According to another aspect of the invention, a digital information distribution system is provided in which a validity of electronic watermark information of a user embedded through an electronic watermarking process is guaranteed through an encryption process, wherein the system comprises inspection means for inspecting the electronic watermark information of the user.
Another feature of the digital information distribution system is that the system further comprises inspection means for inspecting a signature of the user by using an anonymous public key with a certificate issued by a certification facility.
Another feature of the digital information distribution system is that an image format is used which has a header field storing information to be used for inspecting the encryption process and/or electronic watermark embedding process.
According to another aspect of the invention, a storage medium is provided which stores a program for making a computer execute a function of each process of the electronic watermarking method.
According to another aspect of the invention, a storage medium is provided which stores a program for making a computer execute a function of each means of the digital information distribution system.
The embodiments are constituted of the above-described technical means, and only part of encryption keys are multiplied by random numbers. It is therefore possible to prevent processes for encryption keys and random numbers from being reversed. It is therefore possible to prevent a server from making a user use an encryption key intended by the serve.
According another feature of the embodiments, the amount of calculation and communication necessary for embedding an electronic watermark can be reduced as compared to a conventional system and the efficiency of an electronic watermarking method can be improved considerably.
Still other objects of the present invention, and the advantages thereof, will become fully apparent from the following detailed description of the embodiments.